Microcontact printing is a technique of patterning structures on a surface by using an elastomeric stamp with relief patterns. The stamp is inked with the material to be patterned then put in contact with the surface and transfer of the ink only occurs in the areas of the relief that come in contact with the surface.
Current methods of micro-contact printing pose several challenges. One primary difficulty is the need to achieve reliable contact between the stamp surface pattern elements and the substrate. As the “ink” being transferred is essentially a molecular monolayer, it is essential that the stamp (typically made from a polymer such as polydimethylsiloxane (PDMS)) come into intimate contact with the substrate. This can be accomplished in a number of ways. The stamp surface may be taken to a nominal position which is slightly past the substrate thus ensuring that despite the inevitable imprecision of the locations of the stamp surface and the substrate, intimate contact is achieved. However, this method has the disadvantage of either requiring very accurate positional control or of compressing the stamp and thereby creating pattern distortions of unacceptable magnitude including the possibility of establishing contact between the substrate and areas of the stamp between the raised features. As feature size is on the order of 5 microns one can appreciate the difficulty involved.
A more sophisticated approach to the problem controls, or measures and controls, the contact force as the stamp touches the substrate. A number of methods can be employed to achieve this control. A spring or similar device may be inserted into the motion control mechanism to establish an essentially uniform contact force over a sufficiently wide range of linear displacement. Alternatively a pneumatic or hydraulic coupling may be used to the same end or a direct force measurement may be used as feedback to control the motive device.
These methods mitigate the contact issue in that they decrease the required precision of the motion control. However, there are several drawbacks. A simple scheme of spring loading may result in variations in contact force due to friction or variation in mechanical components. Determination of the proper force may be a protracted process as well, requiring careful adjustments of pretension in the spring or replacement of spring elements.
The use of a pneumatic or hydraulic coupling eases the required adjustments, but still presents problems of friction and control. Force feedback to the motion control device allows rapid, controllable adjustment that may essentially compensate for mechanical variations, but adds considerably to the cost of a patterning system. Most importantly, while all these approaches will compensate for errors of position in an axis perpendicular to the planes of the stamp and substrate, none of them provide for angular deviations. If the stamp initially contacts the substrate in a non-parallel manner, continued application of contact force will result in locally greater stamp compression in the area of first contact. If enough force is applied to ensure intimate contact of the entire patterned area, excessive compression of the stamp elements in this initial contact area and subsequent distortion of the pattern may result.
Given careful engineering, design and fabrication of motion control devices, stamps and substrates, and selection of high precision components, many of the problems noted above may be avoided or mitigated. However, achieving this level of precision is expensive and may result in a system that requires a high level of maintenance. For many applications, these conditions may be acceptable, but for circumstances of high volume production or especially if multi-site production is required, a lower cost, more robust method is desirable.